The purpose of a vehicular transmission is to provide a neutral, at least one reverse and one or more forward driving ranges that impart power from an engine, and/or other power sources, to the drive members which deliver the tractive effort from the vehicle to the terrain over which the vehicle is being driven. As such, the drive members may be the front wheels, the rear wheels, both the front and the rear wheels or a track, as selected, to provide the desired interaction with the terrain.
The present invention is not to be confused with a series propulsion system. A series propulsion system is a system in which power follows a path from an engine to an electrical storage device and then to an electric motor which applies the power to rotate the drive members. There is no direct mechanical connection between the engine and the drive members in a series propulsion system.
Transmissions adapted to receive the output power from either an engine or an electric motor, or both, as well as transmissions adapted to receive the output power from either an engine or a hydro-mechanical device, or both, have heretofore relied largely on what has been designated as series, hybrid propulsion systems. Such systems have not, heretofore, accommodated high-average power vehicles nor do they provide duty cycles that demand continuous, constant speed operation. The provision of high tractive effort at desired high efficiencies is not readily achievable with a typical, series, hybrid transmission configuration, particularly in reverse.
The challenge, therefore, is to provide a transmission that will operate at high efficiencies over a wide variety of operating conditions. Desirable electro-mechanical, or hydro-mechanical, transmissions should leverage the benefits of a series, hybrid transmission for desirable low-average power duty cycles--i.e.: low speed start/stop duty cycles--as well as the benefits of a parallel hybrid transmission for high-average output power, high speed duty cycles. In a parallel arrangement the power supplied by the engine and the power supplied by the supplemental source of energy--i.e.: motor/generators or hydrostatic power sources--are independently connected to the drive members. As such, the present invention is not to be confused with a parallel propulsion system. However, should further definition as to that type propulsion system be desired, a detailed explanation as to the structure and operation of variable, two-mode, input-split, parallel, hybrid electro-mechanical transmissions is set forth in pending U.S. patent application, Ser. No. 09/103,850, filed Jun. 24, 1998, which has been assigned to General Motors Corporation.
Moreover, perfecting a concept wherein two modes, or gear trains, are available for synchronous selection by the on-board computer to transmit power from the engine and/or the motor/generators in electro-mechanical transmissions--and from the engine and/or the hydrostatic power source in hydro-mechanical transmissions--to the output shaft results in a hybrid transmission having an extremely wide range of applications. Unfortunately, even such hybrid transmissions have heretofore provided very limited speeds and tractive effort in reverse.
The desired speeds and tractive effort at over-the-road cruising operation may be accomplished by the use of a variable, two-mode, input-split, parallel, hybrid electro-mechanical, or hydro-mechanical, transmission. Such transmissions utilize an input member to receive power from the vehicle engine and a power output member to deliver power to drive the vehicle.
In electro-mechanical variations first and second motor/generators are connected to energy storage devices, such as batteries, so that the energy storage devices can accept power from, and supply power to, the first and second motor/generators. A control unit regulates power flow among the energy storage devices and the motor/generators as well as between the first and second motor/generators. Such an arrangement is depicted in U.S. Pat. No. 5,558,589 issued on Sep. 24, 1996, to General Motors Corporation.
In hydro-mechanical variations hydrostatic devices are substituted for the motor/generators. The present invention is not dependent upon which variation is employed. That is, either electric motor/generator units or hydrostatic pump/motor units may be utilized in two-mode, compound-split, vehicular transmissions employing the concepts of the present invention so as to provide both enhanced speed and tractive effort--even while operating in reverse.
As will become apparent, the present invention is readily adaptable to the military market, the agricultural market or the commercial market.
It is well appreciated that input split transmissions cannot transmit high power in reverse. With commercial versions of input split transmissions--which typically do not deliver high tractive effort in reverse--an accommodation for this drawback has been achieved by supplementing the required input power with batteries while in reverse range. Where high tractive effort is required, or desired, in the reverse range input split transmissions are not generally acceptable. As will be hereinafter more fully described, two-mode, compound-split, vehicular transmissions can be adapted to incorporate the concepts of the present invention and thereby deliver considerably enhanced tractive effort in reverse--and over an enhanced range of speeds.
For the commercial market, one should design the transmission to provide the highest overall efficiency at vehicle cruise speed inasmuch as commercial trucks, for example, travel at such speeds during the vast majority of the time that they are operating.
To the contrary, vehicles intended for the agricultural market spend the vast majority of their operation at speeds falling within the range of from about five to about nine miles per hour. Thus, while agricultural tractors can be driven at over 20 miles per hour, such usage is not normally continuous over an extended period of time. In order to provide the desired tractive effort for the relatively low speeds at which agricultural tractors normally operate, transmissions for such vehicles have increased the engine speed to vehicle speed ratios so as to provide as many as twelve to sixteen speed shifts. However, such transmissions have largely been designed as two and three range, output split, hydro-mechanical transmissions with synchronous shifts. To date no input split or compound-split transmission has been provided for the agricultural market.
It must be recognized that most hydrostatic pump/motor units cannot readily be disposed concentrically about one or more planetary gear sets, and for that reason hydrostatic pump/motor units generally need to be located on their own centerlines that are laterally offset from the common centerline of the planetary gear sets employed in the transmission. To the contrary, electrical motor/generator units are well adapted to being concentrically disposed with respect to the planetary gear sets with which they are used. Except for this accommodation to the lateral disposition of the hydrostatic pump/motor units, the present invention utilizes a two mode, compound-split, arrangement that provides at least one mechanical point in the first range, or mode, and at least one, and (as disclosed) two mechanical points in the second range, or mode.
It should be noted that a mechanical point occurs when either of the supplemental power receiving/delivery units--i.e.: either electro-mechanical (such as motor/generator units) or hydro-mechanical (such as hydraulic pump/motors units)--is stationary at any time during operation of the transmission in either the first or second mode. The lack of a mechanical point is a drawback inasmuch as the maximum mechanical efficiency in the transfer of power from the engine to the output occurs when one of the supplemental power units is at a mechanical point--i.e.: is stationary. In variable, two-mode, input-split, parallel, hybrid electro-mechanical, or hydro-mechanical, transmissions, however, there is typically one point in the second mode at which one of the supplemental power units is not rotating such that all the engine power is transferred mechanically to the output. In the aforesaid U.S. Pat. No. 5,558,589 there are two mechanical points in the second mode as well as one mechanical point in the first mode.
By thus selecting the mechanical path within the transmission, the power and size of the supplemental power units can remain small and yet increase the overall efficiency of the transmission.
For the military market one designs the transmission to provide sixty percent maximum gradeability--i.e.: the inclination of the grade expressed as a percentage between horizontal (zero %) and vertical (100%) that a vehicle can climb without losing speed--versus the twenty-five percent gradeability desired for commercial vehicles. In addition, the top speed for a military transmission is customarily about 55 miles per hour versus about 65 miles per hour for its commercial counterpart. As such, the military version requires approximately twice the output corner horsepower of a commercial transmission for the same input power. Thus, for a 40,000 pound vehicle this would be a maximum tractive effort of 10,000 pounds for a commercial vehicle vis-a-vis 24,000 pounds for the military application.
However, the prior art does not teach or suggest any two-mode, compound-split, vehicular transmission configuration wherein the reverse range is much other than a redirection of the low speed, low tractive effort provided in the initial portion of the low speed, forward range.